This invention relates to polycationic reagents, to novel cationic polysaccharide derivatives produced by reaction of polysaccharides with these reagents, and to the use of these polysaccharide derivatives in paper manufacturing. The polycationic reagents have at least two cationic groups and one polysaccharide-reactive group. Certain of these reagents are novel compositions of matter.
The modification of starch and other polysaccharides by chemical derivatization to produce various cationic polysaccharides is well known. Cationic polysaccharides, i.e., polysaccharides which have been modified so that they have a positive electrostatic charge, are used for a large number of applications and are particularly useful in the manufacture of paper due to their superior performance in the paper production as compared to unmodified polysaccharides. Amphoteric polysaccharides, i.e., polysaccharides which have been modified so they have cationic groups, together with a controlled amount of anionic (e.g., phosphate) groups, are used in a similar manner, with superior performance as compared to unmodified polysaccharides.
As used herein, the term "paper" includes sheet-like masses and molded products made from fibrous cellulosic material, which may be derived from natural sources as well as from synthetics such as polyamides, polyesters and polyacrylic resins, as well as from mineral fibers such as asbestos and glass. Also included are papers made from combinations of cellulosic and synthetic materials.
Various materials, including starch, are added to the pulp, or stock, during the paper-making process, prior to the formation of the sheet. One purpose of such additives is to bind the individual fibers to one another, thus aiding the formation of a stronger paper. Alum is employed in paper-making processes which are conducted under acidic conditions, however, alum-free, alkaline conditions in paper-making processes are becoming common in the industry.
In the case of those papers which contain added pigments, such as titanium dioxide, it has been known to add materials to the pulp, or stock, for the specific purpose of retaining a greater proportion of such pigments in the paper (rather than have them drain off in the water that is removed during the formation of the sheet). Such additives are often referred to as "pigment retention agents." Cationic starches have long been employed as additives in paper production for their contributions to drainage, strength and pigment and fine pulp retention in paper.
Starches used in paper manufacturing are typically used in dispersed form and starch inhibition is avoided. Starch inhibition refers to crosslinking modifications to the starch granule which limit or prevent granule hydration or swelling and disintegration during cooking (gelatinizing) to form a hydrated colloidal dispersion of starch molecules. Inhibited starches cannot form desirable dispersions and are not as effective in paper production as uninhibited starches. Other polysaccharides perform in a similar manner.
The polycationic reagents which have been used in the past to prepare polysaccharide derivatives contain more than one polysaccharide-reactive group or require reaction conditions which provide an opportunity for crosslinking of the polysaccharide. Thus, cationic polysaccharide derivatives prepared from these known reagents are only useful in paper production so long as the crosslinking, or inhibition, is controlled. Unlike the known reagents, the polycationic reagents of the present invention contain only one polysaccharide-reactive site (and therefore are not susceptible to undesirable crosslinking) and polysaccharide derivatives made therefrom are useful in paper production without any need for control of crosslinking of the polysaccharide derivative.
It has now been discovered that unexpected superior performance in paper production may be achieved by the use of novel cationic polysaccharide derivatives which are prepared by reaction of a polysaccharide with a reagent containing a single polysaccharide-reactive group and more than one cationic group. Where the cationic group is an amine, the performance of these derivatives far exceeds their expected performance based on degree of substitution or nitrogen content alone. These advantages are most advantageous in alum-free processes for making paper under alkaline conditions. It is believed that the high charge density per saccharide monomer unit which is produced by reaction with the reagents disclosed herein is responsible for the unexpected improvement in performance.
Accordingly, this invention provides a new class of cationic polysaccharide derivatives and polycationic reagents useful in the preparation of these polysaccharide derivatives. This invention also provides cationic polysaccharides for use in paper manufacturing showing improved drainage, pigment and pulp retention and paper strength as compared to cationic polysaccharides of the prior art.